The present invention relates to a process for preparing aliphatic C4-C18 xcex1,xcfx89-dicarboxylic acids from C6-C20 xcex1,xcfx89-dienes.
Aliphatic xcex1,xcfx89-dicarboxylic acids are important compounds in organic synthesis because of their bifunctionality. The short-chain representatives are important reagents for organic syntheses of all kinds, while the longer-chain representatives are preferred for making polyamides, polyesters, plasticizers, alkyd resins and lubricants.
Special significance attaches to adipic acid, an important starting material for making nylon 66.
In industry, adipic acid is produced from cyclohexane in a two-step process. First, the cyclohexane is oxidized to a cyclohexanol-cyclo-hexanone mixture. This so-called xe2x80x9cKA oilxe2x80x9d is further oxidized, generally with nitric acid, producing stoichiometric amounts of nitrous gases. The alternative use of atmospheric oxygen as oxidizing agent results in lower product selectivity.
A further process for producing adipic acid is the biscarbonylation of butadiene (U.S. Pat. No. 4,259,520), which leads to adipic acid via the hydrolysis of the bisester. The formation of isomeric methyl glutarate, the branched biscarbonylation product, is unavoidable.
Adipic acid has been prepared from cyclohexene using tungstic acid/aqueous hydrogen peroxide solution in tert-butanol. The main disadvantage here was the long reaction time of 24 hours (see T. Oguchi et al., Chem. Lett. 1989, 857-860).
It is an object of the present invention to provide easy and selective access to adipic acid via xcex1,xcfx89-dienes as readily available reactants.
It is known that xcex1,xcfx89-dicarboxylic acids can be prepared by oxidative cracking of xcex1,xcfx89-dienes or cyclic alkenes. The oxidizing agents used are ozone, potassium permanganate, chromates or peroxides.
Antonelli et al., J. Org. Chem. 63, 1998, 7190-7206 describe the conversion of 1,7-octadiene into adipic acid in the presence of aqueous hydrogen peroxide solution and methyltrioctylammonium tetrakis-(oxodiperoxotungsten)phosphate as phase transfer catalyst. This process, however, provided an adipic acid yield of only 60%.
The stated object of the invention is achieved by such a process for preparing optionally substituted aliphatic C4-C8 xcex1,xcfx89-dicarboxylic acids by oxidation of optionally substituted C6-C20xcex1,xcfx89-dienes with hydrogen peroxide or compounds which release hydrogen peroxide under the reaction conditions as is characterized in that the reaction is carried out in the presence of transition metal catalysts of group VI and of a mixture comprising at least one organic acid and at least one aprotic organic solvent. The process of the invention can also be used to produce adipic acid. The process of the invention is notable for providing the desired reaction products selectively and in very high yields within short reaction times.
In a preferred embodiment of the process according to the invention, the organic acids used are straight-chain or branched C2-C8 alkylcarboxylic acids, C6-C14 arylcarboxylic acids or C7-C15 arylalkyl-carboxylic acids which optionally bear at least one further substituent selected from the group consisting of halogen atoms, straight-chain or branched C1-C6 alkyl groups, C3-C7 cycloalkyl groups and carboxyl groups. Examples thereof are acetic acid, propionic acid, pivalic acid, chloroacetic acid, benzoic acid, terephthalic acid and phenylacetic acid. Particular preference is given to using acetic acid. In a further preferred embodiment of the process according to the invention, mixtures of organic acids can be used as well.
In a preferred embodiment of the process according to the invention, the aprotic organic solvent used is dioxane, ethyl acetate, propyl acetate, butyl acetate, chlorobenzene, acetonitrile or benzonitrile. Particular preference is given to using dioxane.
In a further preferred embodiment of the process according to the invention, mixtures of aprotic organic solvents can be used as well.
The process of the invention is preferably carried out using the organic acid in a ratio of 1:5 (v/v) to 5:1 (v/v) to the aprotic organic solvent, particular preference being given to a ratio of 1:2 (v/v) to 2:1 (v/v), and a ratio of 1:1 (v/v) is most preferred.
In a particularly preferred embodiment of the process according to the invention, the mixture of at least one organic acid and at least one aprotic organic solvent is a single-phase mixture.
The reactants used in the process of the invention are C6-C18 xcex1,xcfx89-dienes, e.g., 1,7-octadiene, 1,9-decadiene or 1,11-dodecadiene, which may optionally bear at least one substituent selectable from the group consisting of C1-C6 alkyl groups, e.g., methyl, ethyl, propyl or isopropyl groups, C6-C22 aryl groups, e.g., phenyl or naphthyl groups, C7-C24 arylalkyl groups, e.g., benzyl or phenylethyl groups, and halogens, e.g., chlorine or bromine. The reagent used is particularly preferably 1,7-octadiene, which the process of the invention converts into adipic acid.
In a further preferred embodiment of the process according to the invention, mixtures of C6-C,18 xcex1,xcfx89-dienes can be used as well. The C4-C18 xcex1,xcfx89-dicarboxylic acid mixtures obtainable by the process of the invention are useful for polyester production, for example.
In a preferred embodiment of the process according to the invention, the transition metal catalyst of group VI is a tungstic acid, a tungstate or a tungsten complex. Particularly preferred tungstates are tungstates of oxidation states II, IV and VI, and most preference is given to using Na2WO4 or (NH4)2WO4. A preferred tungsten complex is the peroxo complex K2WO8. 
The hydrogen peroxide is preferably used in the process of the invention as a 5% to 50% (w/v) aqueous hydrogen peroxide solution, particularly preferably as a 30% to 50% (w/v) aqueous hydrogen peroxide solution.
Preferred compounds for releasing hydrogen peroxide under the reaction conditions are percarbonates, peroxyacids or perborates.
The process of the invention is customarily carried out using 9 to 20 equivalents of hydrogen peroxide based on the xcex1,xcfx89-diene used. Preference is given to using 10 to 13 equivalents.
The process of the invention is customarily carried out within the temperature range from about 40xc2x0 C. to about 150xc2x0 C., preferably a temperature that ranges from about 50xc2x0 C. to about 120xc2x0 C.
The process of the invention is particularly preferably carried out along a temperature profile. In this case, the temperature is maintained within the range from about 50xc2x0 C. to about 75xc2x0 C. at the start of the process of the invention and is raised to a temperature that ranges from about 85xc2x0 C. to about 120xc2x0 C. after 30 to 90 minutes.
In a preferred form of the process according to the invention, the equivalents of hydrogen peroxide are added to the reaction mixture in two stages. Initially, 1.5 to 3.5 equivalents of hydrogen peroxide are added to the reaction mixture, based on the xcex1,xcfx89-diene used, while the temperature is maintained within the range of from about 50xc2x0 C. to about 75xc2x0 C. for 30 to 90 minutes. The temperature is then raised to a temperature that ranges from about 85xc2x0 C. to about 120xc2x0 C. and the remaining 5.5 to 18.5 equivalents of hydrogen peroxide are added. In a particularly preferred form of the process according to the invention, initially 2.5 equivalents of hydrogen peroxide are added at about 65xc2x0 C. and the temperature is maintained for 60 min, and then the temperature is raised to about 95xc2x0 C. and the remaining 9.5 equivalents of hydrogen peroxide are added.
The two-stage addition of hydrogen peroxide and the temperature profile lead to an increase in the selectivity with regard to the conversion into the desired xcex1,xcfx89-dicarboxylic acid.
The invention is further described in the following illustrative examples in which all parts and percentages are by weight unless otherwise indicated.